Today I was told a rule about inserting conditional if in a sentence.
I was told that when we insert if to a present sentence we use the second case, and we mustn't use the first. Is this correct?

Because I normally teach my students that if the sentence is in the present and likely to happen they should use the first case, but if it is unlikely they should use the second.

Ex. Rewrite the following:
1. She studies well, she gets the full mark. (Use: If)
So for example they told me this question must be answered like this
If she studied well, she would get the full mark
not like this
If she studies well, she will get the full mark

I would really appreciate if you can tell me if that rule is correct or should I continue teaching it the way I know?
Thanks!

I'm a bit confused about who has told you what, or what you mean by "first case" and "second case". But as far as I gather, your idea is right. Both examples are possible, but used in different situations:

If she studies well, she will get the full mark First Conditional for Real Possibility

If she studied well, she would get the full mark Second Conditional for Less Real Possibility